The invention relates to damping units, and more particularly, to a damping unit adapted to suppress vibration at the distal end of a cantilevered rotating hollow shaft used for positioning, such as the distal end of a hand of a robot.
When a cantilevered structural body such as the distal end of a robot hand performs the positioning operation, low-frequency residual vibration caused at the distal end at the time the positioning operation has been stopped is often considered as a problem. Persistence of such residual vibration leads to unstable stoppage, so that it becomes time-consuming to put the robot hand in stability. As a result the working cycle is increased. To overcome this problem, the static rigidity of the structural body is usually increased to control the vibration. However, an increase in the static rigidity of the cantilevered structural body increases the weight of the apparatus as a whole, which is not desirable.
If a damper is used to control vibration, the problem of increased weight becomes substantially negligible. A conventional vibration absorbing damper of this type is disclosed in, e.g., Japanese Utility Model Unexamined Publication No. 1-138636. FIG. 15 shows such vibration absorbing damper 1 applied to a deburring device attached to the distal end of an arm of a robot. A first flange 4 is fixed on a support cylinder 3 that rotatably supports a tool 2, the first flange 4 being made of a vibration absorbing member such as urethane rubber. A second flange 7 is mounted on the first flange 4 by interposing bolts 5 therebetween to provide a distance in the axial direction. A notch 8 is arranged in a central portion of the second flange 7, so that the support cylinder 3 can move as the first flange 4 is deformed. The deburring operation is performed with the tool 2 by first attaching the vibration absorbing damper 1 to the second flange 7 that is secured to the distal end of the robot hand and then rotating a drive shaft 9. Since the vibration of the tool 2 is absorbed by the rubber material of the first flange 4, the vibration transmitted to the robot body can be reduced.
Further, Japanese Patent Unexamined Publication No. 2-279293 proposes an idea that a vibration absorbing damper is mounted on a speed reducing mechanism for a robot. FIG. 16 shows a joint of the robot to which the speed reducing mechanism is applied. A rotating machine 12 is secured to a joint support portion 11, and one end of a robot arm 16 is rotatably mounted on a rotating shaft 12a of the rotating machine 12 through first and second speed reducers 13, 14 and a damper 15. The speed reduction ratios of both first and second speed reducers are equal. Each of the speed reducers 13, 14 is of, e.g., a flat harmonic drive type. The fixed-side discs of both speed reducers are fixed on the joint support portion 11, and the input shafts thereof are connected to the rotating shaft 12a. The output shaft of the first speed reducer 13 is connected to a rotatably supported damper load 17; the output shaft of the second speed reducer 14 is connected to the robot arm 16; and the robot arm 16 and the damper load 17 are coupled to each other by the disc type rotating damper 15. The resonance frequency caused by the first speed reducer 13 and the damper load 17 is set to a frequency higher than that caused by the second speed reducer 14 and the robot arm 16. The rotating damper 15 is of such a structure that a partitioning disc and an orifice disc are combined so as to alternate with each other and a viscous fluid such as machine oil is sealed in the inside thereof. When the two discs rotate relative to each other, the viscous resistance grows as the viscous fluid passes past the orifice, thereby functioning as a rotating damper. By connecting the damper load 17 to the robot arm 16 with this rotating damper 15, damping effects act on the relative movement of the damper load 17 and the robot arm 16 produced in the vicinity of the resonance frequency of the robot arm 16, thereby controlling the vibration of the robot arm 16.
However, the conventional vibration-controlling damper has addressed the problem that low-frequency vibration cannot be sufficiently suppressed.
While a dynamic damper is known as a damper that can effectively control low-frequency vibration, no such dynamic damper as to be applicable to a rotating hollow shaft such as a robot hand has not heretofore been proposed. Therefore, dynamic dampers have not been used for robot hands.